Re: [PATCH 0/6] RISC-V: Add eMMC support for TH1520 boards

["Lad, Prabhakar" <prabhakar.csengg@gmail.com>]

On Wed, Oct 4, 2023 at 5:03 PM Lad, Prabhakar
<prabhakar.csengg@gmail.com> wrote:
>
> On Wed, Oct 4, 2023 at 3:18 PM Robin Murphy <robin.murphy@arm.com> wrote:
> >
> > On 04/10/2023 3:02 pm, Icenowy Zheng wrote:
> > [...]
> > >>>> I believe commit 484861e09f3e ("soc: renesas: Kconfig: Select the
> > >>>> required configs for RZ/Five SoC") can cause regression on all
> > >>>> non-dma-coherent riscv platforms with generic defconfig. This is
> > >>>> a common issue. The logic here is: generic riscv defconfig
> > >>>> selects
> > >>>> ARCH_R9A07G043 which selects DMA_GLOBAL_POOL, which assumes all
> > >>>> non-dma-coherent riscv platforms have a dma global pool, this
> > >>>> assumption
> > >>>> seems not correct. And I believe DMA_GLOBAL_POOL should not be
> > >>>> selected by ARCH_SOCFAMILIY, instead, only ARCH under some
> > >>>> specific
> > >>>> conditions can select it globaly, for example NOMMU ARM and so
> > >>>> on.
> > >>>>
> > >>>> Since this is a regression, what's proper fix? any suggestion is
> > >>>> appreciated.
> > >>
> > >> I think the answer is to not select DMA_GLOBAL_POOL, since that is
> > >> only
> > >
> > > Well I think for RISC-V, it's not NOMMU only but applicable for every
> > > core that does not support Svpbmt or vendor-specific alternatives,
> > > because the original RISC-V priv spec does not define memory attributes
> > > in page table entries.
> > >
> > > For the Renesas/Andes case I think a pool is set by OpenSBI with
> > > vendor-specific M-mode facility and then passed in DT, and the S-mode
> > > (which MMU is enabled in) just sees fixed memory attributes, in this
> > > case I think DMA_GLOBAL_POOL is needed.
> >
> > Oh wow, is that really a thing? In that case, either you just can't
> > support this platform in a multi-platform kernel, or someone needs to do
> > some fiddly work in dma-direct to a) introduce the notion of an optional
> > global pool,
> Looking at the code [0] we do have compile time check for
> CONFIG_DMA_GLOBAL_POOL irrespective of this being present in DT or
> not, instead if we make it compile time and runtime check ie either
> check for DT node or see if pool is available and only then proceed
> for allocation form this pool.
>
> What are your thoughts on this?
>
Something like the below:

diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
index f2fc203fb8a1..7bf41a4634a4 100644
--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h
@@ -198,6 +198,7 @@ int dma_release_from_global_coherent(int order,
void *vaddr);
 int dma_mmap_from_global_coherent(struct vm_area_struct *vma, void *cpu_addr,
                size_t size, int *ret);
 int dma_init_global_coherent(phys_addr_t phys_addr, size_t size);
+bool dma_global_pool_available(void);
 #else
 static inline void *dma_alloc_from_global_coherent(struct device *dev,
                ssize_t size, dma_addr_t *dma_handle)
@@ -213,6 +214,10 @@ static inline int
dma_mmap_from_global_coherent(struct vm_area_struct *vma,
 {
        return 0;
 }
+static inline bool dma_global_pool_available(void)
+{
+       return false;
+}
 #endif /* CONFIG_DMA_GLOBAL_POOL */

 /*
diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
index c21abc77c53e..605f243b8262 100644
--- a/kernel/dma/coherent.c
+++ b/kernel/dma/coherent.c
@@ -277,6 +277,14 @@ int dma_mmap_from_dev_coherent(struct device
*dev, struct vm_area_struct *vma,
 #ifdef CONFIG_DMA_GLOBAL_POOL
 static struct dma_coherent_mem *dma_coherent_default_memory __ro_after_init;

+bool dma_global_pool_available(void)
+{
+       if (!dma_coherent_default_memory)
+               return false;
+
+       return true;
+}
+
 void *dma_alloc_from_global_coherent(struct device *dev, ssize_t size,
                                     dma_addr_t *dma_handle)
 {
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 9596ae1aa0da..a599bb731ceb 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -235,7 +235,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
                 * If there is a global pool, always allocate from it for
                 * non-coherent devices.
                 */
-               if (IS_ENABLED(CONFIG_DMA_GLOBAL_POOL))
+               if (IS_ENABLED(CONFIG_DMA_GLOBAL_POOL) &&
dma_global_pool_available())
                        return dma_alloc_from_global_coherent(dev, size,
                                        dma_handle);


Cheers,
Prabhakar

> [0] https://elixir.bootlin.com/linux/v6.6-rc4/source/kernel/dma/direct.c#L238
>
> > and b) make it somehow cope with DMA_DIRECT_REMAP being
> > enabled but non-functional.
> >
> DMA_DIRECT_REMAP config option is selected by NONCOHERENET config option anyway.
>
> Cheers,
> Prabhakar